The present invention is more especially applied, although not exclusively, to a structural panel or a lining panel of an aircraft, in particular of a rotorcraft. It is known that such a panel generally exhibits, in addition to its structural or lining function, a soundproofing function, in particular when it is mounted in the cockpit or the passenger cabin of said aircraft, where noise which is liable to be very troublesome for pilots and/or passengers is generated.
To this end, it will be noted that, on a rotorcraft, in particular a helicopter, the acoustic spectra defined in the range lying between 20 Hz and 20 kHz arise from the superposition of noise of different origins, which may be placed in two different groups depending on their spectral characteristics, namely pure sounds or discrete frequency noise and broadband noise.
In the known way, pure sounds or discrete frequency noise occurs in particular, as appropriate:                at the characteristic frequencies of the aircraft driveline;        at the rotational frequencies of the rotor blades (main and tail) and at the harmonics of these frequencies;        at the rotational frequencies of the blades of the compressors of the turbine engine units; and/or        at the rotational frequencies of the blades of the blowers that cool the main gearbox or distribute cabin air and/or of electrical equipment, and at the harmonics of these frequencies,while broadband noise comprises, in particular, as appropriate:        the noise of the boundary layer that develops along the fuselage;        the noise generated by the rotors;        the noise of the air intake and nozzle flows;        the engine noise; and/or        the noise of the circuits that provide the cockpit or the passenger cabin with climate control or heating.        
All this noise, and more especially the discrete frequency noise, is very troublesome for the pilots and the passengers.
This is why, on helicopters, provision is often made for various technical means which are associated with structural or lining panels, to reduce the vibratory level or the acoustic radiation of sources of noise and/or of the fuselage of the helicopter, in order to reduce internal noise. To this end, various physical actions can be implemented by these technical means, in particular:                damping of the vibrations of the panels;        attenuation of the acoustic transmission of the panels;        acoustic absorption, by bonding a sound-absorbent foam cladding onto a face of a panel;        a double partition effect between a structural panel and an associated lining panel; and        absorption by “Helmholtz resonator effect”, by perforating the panels.        
The first four physical actions listed above make it possible to reduce the overall noise level in a broad range of frequencies, but lead to a significant and highly disadvantageous increase in mass. In addition, the obtained reduction in noise is not selective enough to eliminate the acoustic annoyance specific to the emergence of the pure sounds.
By contrast, the fifth and final physical action listed above makes it possible effectively to reduce narrowband noise, but still only in a narrow band of frequencies, defined during design.
These various solutions are therefore not entirely effective, in particular for the discrete frequency noise generated by vibratory excitations.
A solution combining one or more of these physical actions advocates the creation of passive soundproofings in the form of lining panels mounted in the cockpit or in the passenger cabin. These panels are designed as a function of the structural region to be treated and of the frequency spectrum to be attenuated.
However, this last solution also has numerous drawbacks and in particular:                the noise reduction is limited, especially at low frequencies;        the increase in mass is high, and may be several hundreds of kilograms in the case of a large helicopter;        there is a not insignificant loss of volume, especially when using thick panels with a view to improving the acoustic absorption effect; and        there are acoustic leakages, particularly at the wiring holes and the joints between the panels.        
In consequence none of the abovementioned known solutions is entirely satisfactory in reducing the annoyance caused by noise, particularly discrete frequency noise.